The present invention relates to a quarternary phase shift keying demodulator.
Because of the large information carrying capability of PSK modulation a compared with frequency modulation for a given period of time over a predetermined bandwidth, PSK modulation techniques have been extensively used in recent years for transmitting digitized audio signals over broadcast satellite links and other transmission facilities.
For demodulating a quarternary PSK modulated signal in which the carrier is suppressed, a local carrier must be regenerated in precise phase relation with the original carrier to permit synchronous detection.
Over the past years, PSK demodulators have been designed on techniques which deal with signals in the carrier frequency band. These techniques include frequency multiplication, remodulation and inverse modulation. Another type of PSK modulation circuit is based on techniques which deal with signals in the baseband frequency such as the Costas loop system and the modified Costas loop system. Cost reduction in multiplier circuits as a result of recent advances in integrated circuits have prompted the use of analog multipliers in greater numbers, with the result that the Costas loop system and the modified Costas loop system have become dominant in the design of PSK demodulator circuitry.
Since the PSK demodulator of the Costas loop system or its modified version basically operates in a phase locked loop, it is important that the elements of the phase locked loop must exhibit excellent stability. However, analog multipliers have met with difficulty in operating at a frequencies higher than 1 Megaherz.